DESCRIPTION: The goal of this proposal is to determine how two key pathways of bile acid biosynthesis are regulated by dietary cholesterol and to determine the mechanisms regulating the apical Na+ -dependent bile-acid transport protein (ASBP) in the ileum. This proposal combines approaches and animal models that have not been combined before to test some basic hypotheses regarding the metabolism and excretion of cholesterol. The proposal has the following goals: 1). To test the hypothesis that the two pathways of bile acid synthesis, the cholesterol 7alpha-hydroxylase pathway and the sterol 27-hydroxylase pathway, are regulated independently, and patterns of regulation depend on cholesterol responsiveness. This will be tested by three different approaches and will take advantage of a unique hypercholesterolemia-resistant rabbit developed at the University of Texas. The applicant proposes a longitudinal study using deuterated 7-alpha hydroxycholesterol and 27-hydroxylated cholesterol as bile acid precursors. These studies will precisely measure rates of bile acid synthesis in vivo as animals are fed a hypercholesterolemic and then a hypocholesterolemic diet. The second component of this specific aim will assess rates of bile acid synthesis by the two pathways in an animal with a total bile acid diversion. The third component of this specific aim is directed a elucidating the mechanism through which these two pathways are regulated. The applicant will examine both steady-state rates of mRNA and use a novel transcription assay to assess how altered rates of enzyme biosynthesis is achieved in vivo. 2). To test the hypothesis that expression of ASBP is regulated by both transcriptional and post-transcriptional mechanisms. This will be tested both in normal and in the hypercholesterolemia-resistant rabbits and the applicant will examine levels in the activity, mRNA, and transcription of the transporter and determine if changes in mRNA are reflected in levels of ASBP protein and localization. In addition, the applicant will determine the functional consequences of ASBP allele found in the hypercholesterolemic rabbit. These studies will make a significant contribution to the understanding of the only routes by which significant cholesterol is removed from the body. In this respect the applicant anticipates that the results will impact upon the treatment of cardiovascular diseases well as having significance for fundamental biological questions of the control of cholesterol homeostasis.